P30 antigen has been detected in the prostate of castrated mice by 3 immunological methods, complement fixation, rocket immunoelectrophoresis and radioimmunoassay. The antigen has not been detected in extracts of prostate from sham-operated animals. Experiments designed to determine the length of time after castration required for appearance of p30 and to investigate the role of hormones in controlling expression of this antigen are in progress.